Electrochemical machining method and apparatus

ABSTRACT

An electrochemical machining technique involves moving a cathode ( 2 ) towards an anodic workpiece ( 1 ). A current is passed through an electrolyte which flows between the cathode ( 2 ) and workpiece ( 1 ) so as to cause material to be removed electrolytically from the workpiece ( 1 ). A vibratory movement is imposed on the cathode ( 2 ) and the current passed between the cathode ( 2 ) and workpiece ( 1 ) is also varied. The vibratory movement may consist of a main sinusoidal oscillation and a secondary ultrasonic vibration, and the current variation is synchronized with the main vibration so that current pulses, and ultrasonic vibration pulses, coincide, with a predetermined small phase shift, with peaks of the main vibration corresponding to the smallest gap between the cathode ( 2 ) and workpiece ( 1 ).

This application claims benefit of PCT/GB00/04033 filed 20 Oct. 2000published as WO 01/30526 which claims benefit of GB 00/04033 filed 23Oct. 1999.

This invention relates to electrochemical machining (ECM).

ECM is a known technique for machining metal workpieces. A cathode isadvanced towards the anodic workpiece in the presence of an electrolyteand a current is passed between the cathode and the workpiece throughthe electrolyte so as to cause material to be removed electrolyticallyfrom the surface of the workpiece.

This technique can be used for the machining of irregularly shapedworkpieces such as dies and moulds, as well as irregularly shaped holesin metals which do not readily yield to mechanical cutting. Also,three-dimensional patterns can be applied to workpiece surfaces derivedfrom a correspondingly shaped cathode.

High currents are desirable to attain high rates of removal of material,and the smaller the gap between the cathode and the workpiece thesharper is the machining definition which can be achieved.

However, with high currents and small gaps there is the problem thatdebris and any operational irregularities can give rise to adverseeffects such as surface roughness, poor accuracy and even damaging shortcircuits. In practice therefore it is necessary to limit the gap to,say, no smaller than 0.2 mm, and this imposes a limitation on thesharpness of definition which can be achieved.

An object of the present invention is to provide an ECM technique withwhich very small gaps can be used with high machining quality, accuracyand productivity.

According to one aspect of the invention therefore there is provided anECM technique wherein a cathode is advanced towards an anodic workpiecein the presence of an electrolyte and a current is passed between thecathode and the workpiece through the electrolyte so as to causematerial to be removed electrolytically from the surface of the materialcharacterised in that vibratory movement is imposed on the cathode so asto cause the gap between the cathode and the workpiece to vary, and thecurrent is also varied.

With this technique it has been found that the vibration of the cathodeand the variation of the current can counter adverse effects of debrisand operational irregularities whereby it is feasible to use muchsmaller gaps and consequently sharper machining definition can beachieved. Gaps down to say 0.01 mm or even 0.005 mm may be feasible,compared with conventionally used gaps down to say 0.2 mm.

The vibratory movement applied to the cathode may comprise a mainvibration preferably a periodic oscillation particularly of a lowfrequency, say in the range 1 to 100 Hz, conveniently of the order of 50Hz. This oscillation may be a sine wave oscillation of constantcharacteristics and preferably it is applied wholly or largely along thedirection of advancement of the cathode towards the workpiece.

With regard to the current variation, this may be of any suitable naturebut preferably occurs on a periodic basis, which may be matched to, andpreferably occurs at the same frequency as, the main vibratory movementof the cathode such that current peaks or pulses are delivered at orclose to positions in the oscillatory cycle of the cathode correspondingto the smallest gap or nearest positioning of the cathode and workpiece.

Most preferably, the current variation has a fixed phase relationshipwith the main vibratory movement of the cathode such that the currentpulses or peaks coincide with, or lag or lead to a predetermined extent,the smallest gap positions in the main vibratory movement cycle.

By arranging for current pulses or peaks to coincide with or be close topositions of maximum convergence between the cathode and workpieceerosion efficiency can be promoted. By arranging for the current todecline, or be switched off, as the cathode moves away from theworkpiece it can be achieved that current flow is commutated therebyminimising stray erosion, which is adverse to accuracy. The periodduring which the gap increases and current flow decreases or terminatesgives an opportunity for debris and machined particles to be flushedaway.

Additionally or alternatively, the vibratory movement applied to thecathode may comprise a secondary vibration preferably of a higherfrequency than the main vibration, generally of the nature of anultrasonic oscillation, particularly having a frequency in the range 10to 60 KHz i.e. 10 to 40 KHz or 20 to 60 KHz. This oscillation may be asine wave oscillation or of any other suitable wave form

This higher frequency vibration can cause cavitation in the electrolytebetween the cathode and the workpiece which dislodges debris and canallow operation with smaller gaps over larger areas without requiringunduly high current levels due to the blocking effect of bubbles. Aneven spread of electrolyte over the cathode and workplace surface can befacilitated. Also the cavitation can help remove metal oxide film andthereby facilitate activation of machining on oxidised metals.

Most preferably this secondary vibratory movement is applied to thecathode simultaneously with the aforesaid main vibratory movement.

Preferably also, the secondary vibration is applied wholly or largelyalong the direction of advancement of the cathode towards the workpiece.

The secondary vibratory movement of the cathode may occur continuouslywith constant, regular characteristics. Alternatively, the vibration,may be discontinuous, and/or may vary or be irregular with regard tofrequency, amplitude, mark-space ratio or any other characteristic asdesired. Thus, by way of example, the secondary vibration can befrequency and/or amplitude modulated and can be applied as individualpulses or as packages of pulses and may be locked to the variation (e.g.frequency) of the electric current and/or to the frequency of the mainvibration.

In a preferred embodiment the secondary vibration movement is tuned inrelation to the cathode's mechanical properties to give resonance.

A control system is preferably provided to effect automatic control ofmachining parameters, and conveniently this system may be computerised.

Thus the control system may control advancement of the cathode asmaterial is removed from the workpiece surface so as to maintain adesired cathode/workpiece gap. This may be achieved by monitoringcurrent and/or voltage characteristics across the gap. Additionally oralternatively other indications may be utilised such as optical oracoustic monitoring of the gap. In the latter respect, where ultrasonicsecondary vibratory movement is applied to the cathode as mentionedabove this can result in the generation of an acoustic signal dependenton the magnitude of the gap and this can be monitored with a transducer.

The control system may also control advancement in relation to adetermined starting reference position so as to achieve a desired depthof machining in the workpiece. This reference position may beestablished by determining the position of the cathode when theworkpiece is contacted by the cathode, preferably at abottom-dead-centre position of vibratory movement of the cathode.

The control system may operate to control advancement of the cathode soas to maintain constant parameters for the gap. Alternatively howeverthe control system may operate to vary the gap depending on factors suchas detected variations in machining conditions, or the stage in themachining process e.g. such that initial machining takes place with alarger gap and final precision finishing takes place with a smaller gap.

Alternatively or additionally the control system may control voltageand/or current across the cathode/workpiece gap so as to maintain adesired rate of machining, which may be a constant rate or a varyingrate. In the latter case, the machining rate may be varied in dependenceon machining conditions and/or stage in the machining process.

Provision may be made for pre-setting the control system in accordancewith different requirements, relating for example to differentmaterials, or different types or characteristics of shapes to bemachined. Provision may also be made for pre-setting other parametersfor this purpose, in particular, parameters of the main and/or secondaryvibration and/or the current variation, as mentioned above.

The control system may also be utilised to monitor and maintain at apredetermined or pre-set value parameters relating to the supply ofelectrolyte, particularly the pressure of the electrolyte.

The electrolyte is preferably caused to flow between the cathode andworkpiece e.g. by pumping from an inlet to an outlet through a vessel orshroud enclosing at least parts of the cathode and workpiece.

The electrolyte may be supplemented by an injected aqueous medium whichmay contain an acid or alkali and/or a salt solution and/or abrasiveparticles.

The invention also provides a machine for use in performing the methoddescribed above comprising a cathode support, a workpiece support, meansfor supplying electrolyte between the cathode and workpiece, means forsupplying current to the cathode and the workpiece, means for advancingthe cathode towards the workpiece, means for applying vibratory movementto the cathode to vary the gap between the cathode and the workpiece,and means for varying the current supplied to the cathode and theworkpiece.

In addition the advancement and vibratory movement of the cathode,provision may also be made for other movements to facilitate machiningof different or larger shapes. Thus, provision may be made for movementin one or more axes transversely to the direction of advancement and/orrotation of the cathode about the direction of advancement.

The invention will now be described further by way of example only andwith reference to the accompanying drawings in which:

FIGS. 1 and 2 are diagrammatic sectional views of a cathode and ananodic workpiece in two relative positions in the performance of an ECMprocess in accordance with one embodiment of the invention;

FIG. 3 is a schematic diagram of one form of a control system used inthe performance of the process;

FIGS. 4-9 are waveforms illustrating different parameters of the processfor different modes of operation;

FIGS. 10-14 are waveforms illustrating further different parameters ofthe process for different modes of operation; and

FIG. 15 is a diagrammatic view similar to FIGS. 1 and 2 showing amodification.

Referring to FIGS. 1 and 2, an ECM process for erosion of the surface ofa metal workpiece is performed within the confines of a vessel or shroud(not shown) containing or enclosing the workpiece 1 and a shaped cathode2.

The workpiece 1 is removably fixed in position on a work table and thecathode 2 is supported above the workpiece so as to be movable upwardsand downwards along a vertical axis (Z). The cathode 2 is also movablein other directions (X, Y, R) as discussed below.

An electrolyte, namely a 5 to 10% aqueous solution of sodium chloride orsodium nitrate, is pumped through jets 3 between the cathode 2 and theworkpiece 1 at a velocity of say 100 m/s.

A DC power supply 4 is connected to the cathode 2 and the workpiece 1,giving a negative potential at the cathode 2 and a positive potential atthe workpiece 1, the potential difference being say 15-20 volts.

The cathode 2 has an upright stem structure 5 with a lower shaped head6. The head 6 is shaped in correspondence with the desired shape to bemachined in the workpiece 1.

The stem structure 5 is mounted for movement of the cathode 2 along thevertical axis Z and appropriate mechanisms 7, 8, 9 are provided foreffecting controlled movement along the vertical axis Z in three modes,namely progressive advancement, vibratory movement up and down(oscillation) at a low frequency, and vibratory movement up and down(oscillation) at a higher frequency. Any suitable mechanisms may beused, for example, a screw drive 7 may be used for progressiveadvancement, a cam driven by a DC motor 8 may be used for low frequencyoscillation, and an electromechanical device 9, such as a piezo crystalor electromagnetic coil or the like may be used for the higher frequencyoscillation.

In addition, the cathode 2 is movable, by appropriate drive mechanisms(not shown) along two, mutually perpendicular horizontal axes X, Y, andalso the cathode 2 is rotatable through a path R about its vertical axisZ. The cathode 2 can therefore be moved towards and away from theworkpiece (along the Z axis), its lateral position can be adjusted(along the X and Y axes) and it can be rotated about a fixed location ofthe axis Z, or a movable location of the axis Z (orbitally).

A computerised control system is provided (as shown in FIG. 3) and thisis connected electrically to the cathode 2 and workpiece 1, a source ofpower 10, the various drive Mechanisms 7, 8, 9 for the cathode 2, a pump11 for the electrolyte, and sensors, namely an electrolyte pressuresensor 12, a gap sensor 13 and a cathode bottom-dead-centre sensor 14.

The control system has input controls 15 whereby operational parameterscan be adjusted and pre-set as discussed hereinafter.

The control system incorporates microprocessor logic boards 16, a powersupply 17 connected to the source of power 10 and which produces poweroutputs for supply to the various powered components, and pulse andphase generator devices 18, 19.

In use, operation is as follows:

In a set up mode, the motor 8 is operated to cause the cam to rotate tobottom-dead-centre, i.e. the lowermost position of the cathode 2, asdetected by the sensor 14. This sensor may comprise a magnetic switchoperated by a steel projection on the cam, or it may comprise adifferent optical or mechanical device.

In this bottom-dead-centre position a set up voltage is applied betweenthe cathode and the workpiece and this is monitored as the cathode 2 isadvanced towards the workpiece 1. As soon as the measured potentialdifference between the cathode 2 and the workpiece 1 collapsesindicating electrical contact, the advance is arrested and the bottomposition of the cathode 2 is recorded as a reference position of theworkpiece.

The system is then set in an operational mode and the cathode 2 israised above the workpiece reference position to define a predeterminedor pre-selected gap between the cathode 2 and the workpiece 1.

The bottom-dead-centre position is also recorded as a pulse referenceposition which is utilised by the phase generator 19 and pulse generator18. That is, when the cam is at bottom-dead-centre a pulse is generatedfrom the sensor 14 which is used by the phase generator 19 to initiatevarious operations, as discussed below, which are thereby locked inphase with the low frequency oscillating positioning of the cathode 2.

The cam is of variable lift (amplitude) and the drive motor 8 can beadjusted to vary the drive speed of the cam and hence the frequency ofoscillation of the cathode 2. These parameters are pre-adjusted tovalues which are maintained constant throughout a particular machiningprocess. The frequency of oscillation of the cathode 2 will be typicallyabout 50 Hz and, being driven by the rotating cam, will be of sine waveform.

During the normal operation mode, the electrolyte is pumped underpressure between the cathode 2 and workpiece 1 and the pump 11 iscontrolled, in relation to the pressure detected with the sensor 12, tomaintain a constant pressure at a pre-selected value. If desired air canalso be pumped into the electrolyte at the same pressure and thisincreases the machine's ability to work on larger surface areas.

The DC current from the power supply 14 fed to the cathode 2 and theworkpiece 1 varies sinusoidally. That is, a pulse of current, or morepreferably a package or short train of pulses having a high frequency ofinterruption, is generated with the shape of the positive half of a sinewave, and this is synchronised with the bottom-dead-centre position ofthe cathode 2, as determined by the phase generator 19. The synchronismmay be such that the pulse is precisely in phase with thebottom-dead-centre position although alternatively it may lag or leadslightly this position (a negative or positive phase value which can bepre-set as desired).

In addition to the low frequency oscillation, the cathode 2 is subjectedto the higher frequency oscillation at a supersonic frequency(ultrasonic), say in the range 20 to 60 KHz. The actual frequencydepends on the length of the cathode 2 and is preferably tuned to obtainoptimum resonance.

The ultrasonic vibration may be applied as a continuous vibration or forintermittent periods in-phase with the main low frequency vibration ofthe cathode 2.

The magnitude of the impulse current applied is monitored andautomatically controlled by measuring the voltage between the cathode 2and the workpiece 1. A voltage is pre-computed in accordance withconditions and requirements and the impedance of the power supply 4 isadjusted so as to achieve the required voltage and consequently themachining current. Higher machining currents give higher rates ofmachining.

The impedance between the cathode 2 and the workpiece 1 can changeduring machining e.g. due to change in workpiece surface area, and thepower supply impedance is automatically adjusted to compensate for thisand thereby maintain the voltage constant.

The cathode 2 is advanced towards the workpiece 1 and this may be setfor progression at a constant velocity equal to the mean speed of anodicdissolution over a period, with servocontrol. The servocontrol isdetermined by monitoring the rising and falling edges of the currentpulses to assess the machining conditions. If the machining conditionsare good and material is being removed readily from the workpiece, acontrol pulse is transmitted to a servo amplifier 20 such as to causethe cathode 2 to advance. If machining conditions are unsuitable forfurther progression no pulse is given to the servo amplifier 20 and thecathode 2 is not advanced.

The duration or extent of machining is determined in relation to theknown reference starting position of the workpiece determined asmentioned above, and the desired depth of machining.

In the event that the cathode 2 approaches the workpiece 1 too closelyat any point to the extent that there is a risk of contact this willinitially be detected by contact between the cathode 2 and workplace 1occurring for a very short period of time due to the movement of thecathode 2 at ultrasonic frequency imposed on the low frequency vibrationand advancement of the cathode. Any such contact causes a momentaryshort circuit which triggers a short circuit monitoring device 21 whichautomatically takes appropriate remedial action such as lifting of thecathode 2 and/or disconnection of power. The extremely short duration ofthe short circuit limits the possibility of damage.

The normal working gap between the cathode 2 and the workpiece 1 i.e.the gap at the bottom-dead-centre position is preselected in accordancewith requirements and is pre-set in relation to the above mentionedinitial bottom reference position, and is then maintained by control ofthe advancement of the cathode by monitoring the voltage drop(particularly a high-frequency component of voltage drop) and/or currentcharacteristics across the gap and/or by measuring the actual size ofthe gap using the gap sensor 13. An acoustic sensor responsive to anacoustic signal generated within the gap by the ultrasonic vibration canbe used.

The gap requirement can be pre-set and varied as desired. A larger gapmay be appropriate in some cases whereas a very small gap may bedesirable in the case where sharp machining of fine detail is required.

With the procedure described above an operator can vary and preselectdifferent parameters to meet the requirements of a particularapplication and this can be done by accessing data in a stored database.Provision may be made for accessing the database during machining sothat parameters can be changed during erosion. This allows for highstock removal at the start of a cycle and a high quality surface finishat the end of a cycle. The change in machining parameters can beprogrammed then implemented automatically.

The axes of movement of the cathode 2 may be in closed loop control atall times, their position being fed back by optical encoders. Theaverage feed rate and therefore machining rate can be monitored so thata statistical analysis of progress can be made and a strategy can beimplemented to optimise the machining parameters.

The foregoing description is concerned with movement along the ‘Z’ axisi.e. along the vertical axis of the cathode 2. Movement is also possibleabout the mutually perpendicular X and Y axes in a horizontal plane, aswell as rotation of the cathode, appropriate drives being provided forthis. Rotation of the cathode facilitates machining of circular holes;X, Y translational movement permits machining of large areas; andsimultaneous rotational and translational movement gives the capabilityof orbital movement to give accurate profiles and parallel sides withincavities, and simple shaped cathodes can be used to manufacturecomplicated shaped cavities.

FIGS. 4 to 9 show typical waveforms for different modes of operation.

FIG. 4 shows the waveform for low frequency oscillation of the cathode(typically approximately 50 Hz).

FIG. 5 shows packages of current pulses which packages are locked so asto coincide in frequency and phase with the low frequency oscillations.

FIGS. 6-9 show the high frequency (ultrasonic) vibration in relation tofour different modes of operation.

With FIG. 6 the high frequency vibration is continuous and of constantfrequency and amplitude.

With FIG. 7 the high frequency vibration is continuous but is amplitudemodulated at a low frequency which is phase-locked to the low frequencyoscillation.

With FIG. 8 the ultrasonic oscillation is amplitude modulated at a lowfrequency which is phase-locked to the low frequency oscillation, with adifferent phase relationship to that of FIG. 7.

With FIG. 9 the ultrasonic oscillation is amplitude and frequencymodulated with both forms of modulation phase-locked to the lowfrequency oscillations.

With the operating conditions of FIG. 6, the continuous, constantultrasonic oscillations provide the following important functions:

give an acoustic signal when the predetermined minimum spacing betweenthe electrodes is established;

ensure even spread of the electrolyte on the machined surface thuseliminating any macro-defects on the surface caused by jets ofelectrolyte;

form cavitation bubbles that partially block the surface of the cathodeallowing machining of larger workpiece surfaces without increasing thecapacity of the process current pulses;

in the case of readily passivating (oxidising) alloys, such as titaniumalloys, activate the process of electrochemical dissolution due tocavitation which mechanically destroys oxide films as well as increasethe current density due to a smaller minimum spacing between theelectrodes.

With the operating conditions of FIG. 7 the decreased ultrasonicvibrations at the current pulses permit high current densities at smallgaps, and the increase ultrasonic vibrations between current pulsesfacilitate flushing away of removed material.

The operating conditions of FIG. 8 are particularly suited for machiningtitanium alloys or steel since the increased ultrasonic vibration at thecurrent pulse helps activate the process by removing oxide film.

The operating conditions of FIG. 9 are suited to the machining of largesurfaces when high amplitude and frequency of ultrasonic vibrations arerequired.

Operating conditions other than those shown in FIGS. 6-9 can be useddepending on requirements and conditions.

In particular FIGS. 10-14 show further typical waveforms for differentmodes of operation.

FIG. 10, like FIG. 4, shows the waveform for low frequency oscillationof the cathode (typically approximately 50 Hz).

FIG. 11, like FIG. 5, shows packages of current pulses which packagesare locked so as to coincide in frequency and phase with the lowfrequency oscillations i.e. such that each package of current pulses isessentially centred on a negative peak of the low frequency oscillationcorresponding to the smallest gap between the electrode and workpiece.

FIGS. 12-14, show three different possibilities for the high frequency(ultrasonic) oscillation. Instead of using a continuous ultrasonicvibration with constant amplitude envelope like FIG. 6, or anamplitude-modulated continuous ultrasonic vibration like FIGS. 7-9, thearrangements of FIGS. 12-14 show packages of ultrasonic vibrationessentially centred on the negative peaks of the low frequencyoscillations.

The processes described above permit accurate high quality machiningwith high rates of productivity. Good shaping and smoothness is readilyachievable.

By way of example, with the mode of operation of FIG. 6, the spacingbetween the cathode and the workpiece can be reduced down toexceptionally small values, say of the order of 0.01 to 0.005 mm, whichpermits machining of very sharp definition at high current density,whilst the ultrasonic energy ensures removal of oxide film.

As mentioned, the ultrasonic energy also advantageously providesprotection against damage by short circuit currents. When contact firsttakes place the duration of this will be no more than one half cycle ofthe ultrasonic frequency and this gives time for remedial action to betaken before damage has occurred.

FIG. 15 shows a modification to the arrangement of FIGS. 1 and 2.

A shaped cathode 22 is moved relative to a workpiece 23, and is vibratedand supplied with current in like manner to the arrangement of FIGS. 1and 2.

However, in order to improve the surface machining quality, reduceroughness, remove the oxide film and activate the electrochemicaldissolution process, further features are incorporated as follows.

The cathode 22 and the surface of the workpiece 23 are, like thearrangement of FIGS. 1 and 2, enclosed within a vessel or shroud 24, andthrough this electrolyte can be pumped from an inlet 25 to an outlet 26.

An additional aqueous medium is injected through an auxiliary inlet 27into the electrolyte flow immediately adjacent to the gap between thecathode and workpiece.

This additional medium may be a concentrated acid or alkali whichsignificantly changes the pH of the electrolyte in the working area.

The additional medium may be an electrolyte with high concentration ofanions to activate the electrochemical dissolution.

The additional medium may be a concentrated electrolyte (acid, alkali orsalt solution) containing up to 30% of abrasive particles.

The combination of pulsed high power electric current and ultrasoundwith chemical and/or abrasive action can give desired machining in ashort period of time.

The additional medium will depend on the workpiece material. Chemicaland abrasive action is particularly effective for machining parts madefrom titanium alloys as well as parts from tungsten or tungsten carbide.Suitable acids are hydrochloric and sulphuric acid. Suitable alkalisparticularly for machining tungsten are sodium and potassium hydroxide.Bromide, iodide, chloride, nitrate ions can be used as salt solution.Abrasive particles may be in the size range 5-50μ.

It is of course to be understood that the invention is not intended tobe restricted to the details of the above embodiment which are describedby way of example only.

Thus, for example, whilst reference has been made throughout to movementof the cathode, where appropriate some or all of the movements may beapplied additionally or alternatively to the workpiece.

What is claimed is:
 1. An electrochemical machining technique wherein acathode is advanced towards an anodic workpiece in the presence of anelectrolyte and an electrical current is passed between the cathode andthe workpiece through the electrolyte so as to cause material to beremoved electrolytically from the surface of the material whereinvibratory movement is imposed on the cathode so as to cause the gapbetween the cathode and the workpiece to vary, and the electricalcurrent is also varied wherein the vibratory movement imposed on thecathode comprises main and secondary vibrations.
 2. The techniqueaccording to claim 1 characterised in that the virbratory movementcomprises main and secondary vibrations.
 3. The technique according toclaim 1 characterised in that the main vibration is a periodicoscillation of 1 to 100 Hz.
 4. The technique according to claim 3characterised in that the periodic oscillation is a sine waveoscillation.
 5. The technique according to claim 3 characterised in thatthe periodic oscillation is applied along the direction of advancementof the cathode towards the workpiece.
 6. The technique according toclaim 3 characterised in that the current is varied with a fixed phaserelationship relative to the main vibration.
 7. The technique accordingto claim 1 characterised in that the current is varied on a periodicbasis matched to the main vibration.
 8. The technique according to claim1 characterised in that the current is varied at a frequency the same asthat of the main vibration such that current peaks or pulses aredelivered at or close to positions in the oscillatory cycle of thecathode corresponding to the smallest gap or nearest positioning of thecathode and workpiece.
 9. The technique according to claim 1characterised in that the secondary vibration is of a higher frequencythat the main vibration.
 10. The technique according to claim 1characterised in that the secondary vibration is an ultrasonicoscillation.
 11. The technique according to claim 10 characterised inthat the ultrasonic oscillation has a frequency in the range 10-60 KHz.12. The technique according to claim 1 characterised in that thesecondary vibration is applied to the cathode simultaneously with themain vibration.
 13. The technique according to claim 1 characterised inthat the secondary vibration is applied along the direction ofadvancement of the cathode towards the workpiece.
 14. The techniqueaccording to claim 1 characterised in that the secondary vibration isapplied to the cathode continuously with constant regularcharacteristics.
 15. The technique according to claim 1 characterised inthat the secondary vibration is of varying characteristics.
 16. Thetechnique according to claim 1 characterised in that the secondaryvibration is amplitude modulated.
 17. The technique according to claim 1characterised in that the secondary vibration is applied as packages ofpulses.
 18. The technique according to claim 1 characterised in that thesecondary vibration is tuned to the cathode's mechanical properties togive resonance.
 19. The technique according to claim 1 characterised inthat a control system is provided which controls advancement of thecathode as material is removed from the workpiece surface so as tomaintain a desired cathode/workpiece gap.
 20. The technique according toclaim 19 characterised in that the control system controls advancementin relation to a determined starting reference position so as to achievea desired depth of machining in the workpiece.
 21. The techniqueaccording to claim 1 characterised in that the electrolyte is caused toflow between the cathode and workpiece.
 22. The technique according toclaim 1 characterised in that the electrolyte is supplemented byaddition of an aqueous medium containing at least one substance selectedfrom acids, alkalis, abrasive particles and salts.
 23. A machineassembly for use in performing electrochemical machining of a workpiececomprising a cathode support, a support for the workpiece, means forsupplying an electrolyte between the cathode and workpiece, means forsupplying an electrical current to the cathode and the workpiece, meansfor advancing the cathode towards the workpiece, means for applyingvibratory movement comprising main and secondary vibrations to thecathode to vary the gap between the cathode and the workpiece, and meansfor varying the electrical current supplied to the cathode and theworkpiece.
 24. The machine assembly according to claim 23 characterisedin that provision is made for movement in one or more axes transverselyto the direction of advancement of the cathode optional furtherincluding means for rotation of the cathode about the direction ofadvancement.
 25. An electrochemical machining technique wherein acathode is advanced towards an anodic workpiece in the presence of anelectrolyte and an electrical current is passed between the cathode andthe workpiece through the electrolyte so as to cause material to beremoved electrolytically from the surface of the material whereinvibratory movement is imposed on the cathode so as to cause the gapbetween the cathode and the workpiece to vary, and the electricalcurrent is also varied on a periodic basis with a phase relationship tothe vibratory movement, wherein the phase relationship is adjustable soas to lag or lead slightly the bottom-dead-centre position by a negativeor positive phase value which can be preset.
 26. The electrochemicalmachining technique according to claim 25 characterised in that thevibratory movement comprises main and secondary vibrations and thecurrent is varied on a periodic basis matched to the main vibration. 27.The electrochemical machining technique according to claim 26characterised in that the main vibration is a periodic oscillation of 1to 100 Hz.
 28. The electrochemical machining technique according toclaim 27 characterised in that the periodic oscillation is a sine waveoscillation.
 29. The electrochemical machining technique according toclaim 27 characterised in that the periodic oscillation is applied alongthe direction of advancement of the cathode towards the workpiece. 30.The electrochemical machining technique according to claim 26characterised in that the secondary vibration is of a higher frequencythat the main vibration.
 31. The electrochemical machining techniqueaccording to claim 26 characterised in that the secondary vibration isan ultrasonic oscillation.
 32. The electrochemical machining techniqueaccording to claim 26 characterised in that the ultrasonic oscillationhas a frequency in the range 10-60 KHz.
 33. The electrochemicalmachining technique according to claim 26 characterised in that thesecondary vibration is applied to the cathode simultaneously with themain vibration.
 34. The electrochemical machining technique according toclaim 26 characterised in that the secondary vibration is applied alongthe direction of advancement of the cathode towards the workpiece. 35.The electrochemical machining technique according to claim 26characterised in that the secondary vibration is applied to the cathodecontinuously with constant regular characteristics.
 36. Theelectrochemical machining technique according to claim 26 characterisedin that the secondary vibration is of varying characteristics.
 37. Theelectrochemical machining technique according to claim 26 characterisedin that the secondary vibration is amplitude modulated.
 38. Theelectrochemical machining technique according to claim 26 characterisedin that the secondary vibration is applied as packages of pulses. 39.The electrochemical machining technique according to claim 26characterised in that the secondary vibration is tuned to the cathode'smechanical properties to give resonance.
 40. The electrochemicalmachining technique according to claim 26 characterised in that acontrol system is provided which controls advancement of the cathode asmaterial is removed from the workpiece surface so as to maintain adesired cathode/workpiece gap.
 41. The electrochemical machiningtechnique according to claim 26 characterised in that the control systemcontrols advancement in relation to a determined starting referenceposition so as to achieve a desired depth of machining in the workpiece.42. The electrochemical machining technique according to claim 26characterised in that the electrolyte is caused to flow between thecathode and workpiece.
 43. The electrochemical machining techniqueaccording to claim 26 characterised in that the electrolyte issupplemented by addition of an aqueous medium containing at least onesubstance selected from acids, alkalis, abrasive particles and salts.44. The electrochemical machining technique according to claim 25characterised in that the current is varied at a frequency the same asthat of the main vibration wherein peaks or pulses are delivered at orclose to positions in the oscillatory cycle of the cathode correspondingto the smallest gap or nearest position of the cathode and workpiece.45. A machine assembly for use in performing electrochemical machiningof a workpiece comprising a cathode support, a support for theworkpiece, means for supplying an electrolyte between the cathode andworkpiece, means for supplying an electrical current to the cathode andthe workpiece, means for advancing the cathode towards the workpiece,means for applying vibratory movement to the cathode to vary the gapbetween the cathode and the workpiece, and means for varying theelectrical current supplied to the cathode and the workpiece on aperiodic basis with a phase relationship to the vibratory movement,wherein the phase relationship is adjustable so as to lag or leadslightly the bottom-dead-centre position by a negative or positive phasevalue which can be preset.
 46. The assembly according to claim 45characterised in that provision is made for movement in one or more axestransversely to the direction of advancement of the cathode optionalfurther including means for rotation of the cathode about the directionof advancement.
 47. The machine assembly according to claim 45characterised in that the means for applying vibratory movementcomprises main and secondary vibrations.
 48. The machine assemblyaccording to claim 47 characterised in that the current is varied at afrequency the same as that of the main vibrations such that currentpeaks or pulses are delivered at or close to positions in theoscillatory cycle of the cathode corresponding to the smallest gap ornearest positioning of the cathode and workpiece.